FIG. 1 is a view schematically showing optical paths to form a microscopic image in an electron microscope according to a related art.
An electron source 101 supplies an electron beam, which is passed through a condenser lens 102 to illuminate a sample 103. After irradiating the sample 103, the electron beam is passed through an objective lens 104 to form a first microscopic image 111, and then, is passed through an intermediate lens 105 to form a second microscopic image 112. Finally, the electron beam is passed through a projection lens 106 to form a third microscopic image 113. According to the electron microscope of the related art, the objective lens 104, intermediate lens 105, and projection lens 106 that are typically called image forming lenses cause relatively large imaging aberrations.
FIG. 2 is a model showing image formation by an electron lens in an electron microscope according to a related art and corresponding mathematical processes.
An electron source (not shown) emits an electron beam through a condenser lens (not shown) to a sample 121. The electron beam is passed through the electron lens 122 to form a diffraction pattern 123 on a rear focal plane, and thereafter, a microscopic image 124 on an image plane. The sample 121 corresponds to the sample 103 of FIG. 1, the electron lens 122 to the objective lens 104 of FIG. 1, and the microscopic image 124 to the third microscopic image 113 of FIG. 1.
An optical system of this type corresponds to a series of mathematical processes shown in an upper part of FIG. 2. Namely, a Fourier transform is carried out for a phase change occurring at a bottom face of the sample 121, to provide a phase change spectrum of the sample 121. With respect to the phase change spectrum, a contrast transfer function of the electron lens 122 is computed to provide an image contrast spectrum corresponding to the diffraction pattern 123 on the rear focal plane. To the image contrast spectrum 123, a Fourier transform is carried out to provide an image contrast corresponding to the microscopic image 124.
When the diffraction pattern 123 is observed with a detector, an intensity distribution may be detectable but a phase is undetectable. Accordingly, the detected diffraction pattern loses phase information. There are problems, therefore, that the intensity distribution and phase information both necessary for reconstructing a microscopic image are not together obtainable and that the diffraction pattern observed with the detector is insufficient to provide a microscopic image through a Fourier transform.
To cope with the problems, a method was proposed to detect phase information from a hologram of a diffraction pattern. To create a hologram, it is necessary to make a diffracted wave transmitted through a sample interfere with a reference wave that is a spherical wave not transmitted through the sample. To form such diffracted wave and reference wave, Japanese Unexamined Patent Application Publication No. Hei-9-80199, for example, discloses an electron beam biprism. A biprism of this type is used for a conventional holography microscope that, overlays a parallel plane wave serving as a reference wave over a diffraction pattern of a sample, and the biprism is arranged after the sample and an objective lens.
On the other hand, in the field of electron beam lithography, studies are being made to improve throughputs of multibeam multicolumn drawing apparatuses. The “multicolumn” means electron beam columns formed at intervals of several millimeters. To provide such electron beam columns, there has been proposed a so-called lotus lens having a plurality of openings formed in a magnetic material in parallel with one another. Operation of this lens has experimentally been confirmed (for example, Hiroshi Yasuda, et al., MCC-PoC (proof of concept) system evaluation, Application to Charged Particle Beam Industry No. 132 Committee, No. 161 Study Meeting Material, Japan Society for the Promotion of Science, pp. 125-128, 2003 and T. Haraguchi, T. Sakazaki, T. Satoh, M. Nakano, S. Hamaguchi, T. Kiuchi, H. Yabara, and H. Yasuda, J. Vac. Sci. Technol. B22, 2004, Multicolumn Cell Evaluation of the proof of concept system).
It is required to realize an electron microscope that detects phase information from a hologram as mentioned above, and according to the phase information and an intensity distribution without using image forming lenses, provides a microscopic image having no imaging aberration due to the image forming lenses.
To form a diffracted beam and a reference beam that are coherent to each other in such an electron microscope, a biprism for splitting an incident electron beam must be arranged in front of a sample. A combined illumination lens arranged between the biprism and the sample must form a parallel wave serving as a diffracted beam from one of the split electron beams and a spherical wave serving as a reference beam from the other of the split electron beams.
The split electron beams from the biprism are distanced from each other only by several millimeters to maintain coherency. Due to this, it has been impossible to make a combined illumination lens capable of conducting separate actions of generating a parallel wave and a spherical wave within such a short distance.